CB Subs RC2 - cannot checkout

Any type of action such as proceeding to payment step or trying to process a coupon sends us to a page with this error:

Please complete the username and password fields.

The user is still logged in at this point.

I found some info about CB Connect, though installed it is unpublished at the moment.

The URL it redirects to (which shows the message is)

index.php?option=com_comprofiler&Itemid=140&cbpact=saveeditbasketintegration&cbpbck=533fc5bd514fef577f0802a2aa071f3ad5d752d09cd1281b0104e7ce&cbpbskt=344&cbpintegration=promotion&lang=en&plugin=cbpaidsubscriptions&view=pluginclass